1. Field of the Invention
This invention relates to a ballast arrangement for fluorescent tubes and particularly to a two-wire ballast arrangement which utilizes the leakage inductance of the transformer to achieve a supply of substantially constant filament voltage to fluorescent tubes supplied by a variable power source for dimming.
2. Description of the Prior Art
Typical fluorescent tubes comprise a sealed cylinder of glass having a heating filament at either end and filled with a gas such as mercury vapor. The supplied voltage is utilized to heat the filaments to a point where a thermionic emission occurs such that an arc can be struck across the tube causing the gas to radiate. The initial radiation given off by gases such as mercury vapor is of a short wavelength principally in the ultraviolet end of the spectrum and thus little visible light is produced. In order to overcome this problem, the inside of the tube is coated with a suitable phosphor which is activated by the ultraviolet radiation, and, in turn, emits visible light of a color that is characteristic of the particular phosphor or mixture of phosphors employed to coat the tube. An important consideration in the operation of such fluorescent tubes is concerned with the fact that in order to sustain the arc across the tubes, the filament voltage must be maintained to a predetermined level. It is maintaining this predetermined voltage level which poses the greatest problem in devising a scheme for dimming the output of the fluorescent tube in an energy-saving, light-dimming arrangement.
As one answer to this problem, the prior art developed a three-wire system where the filament voltage was separated from the variable current used to dim the fluorescent tube. In such a system, a first wire was used to supply variable current to the fluorescent tube. A second wire was used to supply constant filament voltage to the filament of the tube. The third wire was used as a return. Although this arrangement was somewhat successful in accomplishing the required fluorescent tube dimming, the costs from the standpoint of both the device itself and of installation of the device were such that a two-wire system more compatible with present installations was preferable.
Earlier prior art two-wire fluorescent tube dimming arrangements were also developed but these provided only a limited dimming capability beyond which the tube would not function. One problem with these early prior art two-wire dimming arrangements was that the filament voltage to the tube in fact varied as the power supplied to the fluorescent tube was varied. As the current supplied to the fluorescent lamp(s) was reduced, the filament voltage was reduced resulting in damage to the lamps by stripping the emission coating from the cathodes, and eventually extinguishing the lamp current when the filament voltage was reduced below the level required to sustain the arc across the tube.
One prior art solution to these problems is found in U.S. Pat. No. 4,163,925 issued Aug. 7, 1979 invented by Zoltan L. Gyursanszky (now Zoltan Zansky), a co-inventor in the present application. That application is assigned to the same assignee as the present invention. By that invention, a two-wire ballast for fluorescent dimming was provided in which first and second terminals were utilized to provide variable dimming power to the fluorescent tube utilizing a choke coil in conjunction with the transformer. Thus, the choke coil was connected between the first terminal and the tube and a transformer was provided having a primary winding connected between the second terminal and a tap on the choke coil selected to provide substantially constant voltage to the primary winding even as the power to the tube is varied. The transformer had a secondary winding associated with the primary winding to supply filament voltage to the filaments of the tube. While this arrangement provided a solution to many of the problems of the prior art, the necessity for providing the choke coil in the system caused the cost level of the assembled ballast to remain above the point where it would be competitive with other ballasts.